Organic polyisocyanates have been used as lacquers, films, coatings and hot-melt adhesives. Since isocyanate compounds are very reactive toward groups with an active hydrogen such as hydroxyl, carboxyl, amine and the like, it is common to control such reactivity by adding a mono-blocking or masking agent to the isocyanate (U.S. Pat. No. 3,115,479 to Windermuth et al.) and then reacting the blocked isocyanate with a polyester containing free hydroxyl groups by heating the mixture to deblock the isocyanate.
As seen in U.S. Pat. No. 2,777,881, it is possible to avoid the use of blocking agents by limiting the amount of isocyanate reacted with terminal labile hydrogen groups of a polyester or polyesteramide so as to afford a material that is in an uncured state. Additional isocyanate groups then are added to the uncured product so that a subsequent irreversible cross-linking reaction with moisture can take place to produce the final cured state with appropriate physical properties.
Another solution that avoids premature introduction of moisture into the product is to use a packaging system to protect the isocyanate from moisture prior to use. Adhesives Age, September 1987, p. 42-43.
U.S. Pat. No. 4,166,873 to Gilliam et al discloses improved hot melt adhesives and coatings formed by adding diisocyanate to polyesters. The inventors note that the incorporation of isocyanate into the polyester molecules does not involve chain-extension or significant cross-linking. U.S. Pat. No. 2,982,754 to Sheffer et al. and U.S. Pat. No. 2,876,725 to Buck at al. (example 4) contain additional examples of polyesters modified by the addition of isocyanates.
U.S. Pat. No. 3,503,927 to Chang et al pertains to a cross-linked network structure where the cross-linking is labile to heat and provided by the reaction between a phenolic group and an isocyanate group. U.S. Pat. No. 3,684,769 to Abbott et al. pertains to thermally reversible polyester or polyether urethane polymers with thermally reversible urethane links between polymer chains. U.S. Pat. No. 4,201,853 to Henry et al reveals a thermally-reversible polymeric binder for plastic bonded explosives that reversibly dissociated below 150.degree. C. Wagener and Muria, Polymer Preprints, Vol. 30, No. 1, April 1989 disclose monomeric thermally reversible urethanes whose molecular weight is a function of temperature. Although a polyurethane was prepared, no discussion or data on polymer urethane bond reversibility are given nor are suggestions made as to its applicability.
U.S. Pat. No. 4,608,418 to Czerwinski et al. illustrates an attempt to improve the performance of conventional isocyanate materials by adding a reactive plasticizer to a hot-melt composition formed from a mixture of one or more polyisocyanates and one or more hydroxyl terminated polyols and one or more chain extenders.
Prior-art isocyanate-based polymers have been low molecular weight isocyanate compositions that afford good working properties, e.g., application ease, surface wettability and penetration, leveling ability, and gap-filling capacity. Such materials are commonly moisture-cured to form substituted polyureas after being applied to give durable coating or adhesive materials. However, such materials do not have the high performance characteristics of some of the more costly high-performance polymers such as the polyimides. Typically as one attempts to improve the performance characteristics of the isocyanate-based materials using conventional techniques, high-viscosity and associated low wettability result in a loss of substrate bonding ability. Currently conventional isocyanate polymers do not allow for the high temperature processing, e.g., soldering and thermoforming, of flexible circuit boards and other components such as chips, transformers and motors. Conventional isocyanate polymers typically do not provide cracking resistance at high end-use operating temperatures such as found in high performance aircraft, automotive and computer equipment. The processibility of high performance materials such as polyimides that are used in high performance protective dielectric film or coating materials is more limited than desired. A need continues to exist for a better, melt-processible, high-performance material such as a polyimide for molding applications.